The spread of cancer cells from a primary tumor site to distant organs is known as metastasis. The progression of human cancer to metastatic disease is the major contributing factor to its lethality. Metastasis has been considered one of the most intriguing aspects of the pathogenesis of cancer. Cancer tumor metastasis, or otherwise known as metastatic disease, is responsible for most therapeutic failures in treating the disease, as patients succumb to the multiple tumor growth, accounting for more than 90% of human cancer related deaths. See, for example, Cancer, A Comprehensive Treatise, F. F. Becker (editor), Volume 4, Chapter 3, Plenum Press, New York, 1975.
In order for a tumor to form lethal metastases it must acquire the ability to carry out a complex series of steps. These steps include: gaining access to the vasculature or lymphatic system (intravasation), surviving during transit, exiting the vascular or lymphatic channels (extravasation), and proliferating at the metastatic site. One of the rate limiting steps in the proliferation of tumors, both at the primary and metastatic sites, is the acquisition of the angiogenic phenotype (Folkman, 1971). The induction of angiogenesis not only allows tumors to grow beyond the size limitation imposed by the diffusion limit of oxygen, but also provides a conduit through which the tumor cells can travel and colonize distant organs (Brown et al., 1999; MacDougall and Matrisian, 1995). Once the tumor cells arrive at the metastatic site they must also induce neovascularization in order to grow beyond a microscopic size. It has been documented, however, that metastatic colonies can remain in a microscopic or dormant state and not progress beyond this size for months or years following the initial colonization (Fidler, 2003).
The presence of dormant or micro-metastases indicates that tumor growth and proliferation is not governed solely by cell-autonomous processes and that the conditions present in the microenvironment that permitted proliferation at the primary site can not exist at the metastatic site. Thus, the ability of a tumor to communicate with the surrounding stroma, composed of fibroblasts, immune cells and endothelium must be reestablished upon arrival at the metastatic site. One way in which heterotypic tumor-stromal signaling could affect tumor growth is through the regulation of the production and secretion of pro- and anti-angiogenic proteins by the surrounding stromal fibroblasts and endothelial cells.
The molecular and genetic events that facilitate escape from the primary site and homing to the metastatic site have been well studied. It has been demonstrated in a murine model of breast cancer metastasis that escape from the primary site was largely dependent on the activity of the transcription factor Twist (Yang et al., 2004). Furthermore, microarray analyses of metastatic human breast cancer cells, derived by serial injection into immuno-compromised mice, revealed sets of genes whose expression correlated with their preferred metastatic destination of hone or lung (Kang et al., 2003; Minn et al., 2005). These studies, though yielding key insights into two critical steps of tumor metastasis, namely intravasation and homing, did not address the requirements for tumor establishment and growth at the metastatic site.
It has been previously demonstrated that tumor cells can stimulate the expression of the pro-angiogenic protein VEGF in the surrounding stroma (Dong et al., 2004; Fukumura et al., 1998). However, the regulation of Thrombospondin (Tsp-1), one of the most potent endogenous anti-angiogenic proteins, in the tumor-associated stroma have not been as well studied (Kalas et al., 2005).
New research into the cell-to-cell signaling events between metastatic tumors and their surrounding stroma can yield novel strategies for treating metastatic disease. There is still a need for methods of treating metastatic disease that have less systemic toxicity than the current standard treatments comprising chemotherapy and/or radiation therapy.